1. Field of the Invention
The present invention relates to and in particular to a method for managing data in a preferably non real-time sensor network.
2. Description of the Related Art
Sensor networks show a constant growth in use and are in particular used in the areas of environment monitoring, for example, to analyze the evolution of the weather, distribution of humidity or water contaminations or in order to measure the temperature on surfaces, to analyze movement patterns, to control big industrial sites etc. The list of application possibilities could be continued almost endlessly.
In case of single sensors of a sensor network, they are wirelessly communicating sensor nodes, which in general consist of a probe, a processor unit, a communication device, as well as an energy source, for example, a battery or solar cells. The functionality of data acquisition, communication and processing are all gathered on a very dense space on the sensor node. This miniature design is extremely advantageous for specific applications, for example, the said environment monitoring, because they enable the deployment of the sensor nodes and consequently an application of the network also in areas that are hard to access.
Critical parameters, which circumstantially could restrict the application possibilities of sensor networks, are in particular given physical values of the individual sensor nodes, for example, their transmission range, processor power, battery capacity, available storage capacity and the like. Due to these physical restrictions, the energy-efficient organization of the sensor network is of particular importance.
In modern sensor networks of this described kind, it is already known to form clusters, wherein within one cluster a sensor node is assigned to be the aggregator node, at which the sensed data of the rest of the sensor nodes of the cluster are aggregated. The election of aggregator nodes can, for example, be performed according to pre-definable criteria, wherein it can be envisioned in particular to elect aggregator nodes depending on the energy resources that are still available for the respective sensor nodes. For this purpose, for example, the LEACH protocol (low energy adaptive clustering hierarchy) can be used together with a simple downstream routing protocol. The LEACH protocol is described in detail in W. B. Heinzelman, A. P. Chandrakasan, H. Balakrishnan, “An Application-Specific Protocol Architecture for Wireless Microsensor Networks”, in IEEE Transactions on Wireless Communications, vol. 1, no. 4, pp. 660-670, October, 2002.
Regarding the request frequency of data sensed by individual sensor nodes and stored in the network, two different types of sensor networks can be distinguished: First, there are so-called non real-time sensor networks basically characterized in that the sensed data is continuously and under real-time requirements transmitted to a central authorized entity. Pre-requisite is a constant access of the entity to the network. If this access gets lost, the system is—until a potential repair—useless. In contrast, for specific applications it is not wanted or simply not possible to read out the sensed data under real-time requirements. Such networks are called non real-time sensor networks. With this kind of networks, the sensed data first has to be stored within the network, until it can be read out at certain temporal intervals by a reader (Reader R) authorized for doing so, for example in the form of a laptop.
With respect to the fact that in non real-time networks the authorized reader is not in constant contact to the network, there are several very serious problems regarding the data management in such networks. On the one hand, the restricted storage capacities and power resources of the individual sensor nodes need to be considered very carefully.
On the other hand, it has to be considered that not only individual sensor nodes can break down, for example, because their power resources have been consumed. Moreover, in non real-time networks it can happen under certain circumstances that after a long operation time without data request complete areas of the sensor network can break down, for example, due to external influences, which in the following will be generally referred to as “disaster”. In extreme cases, such a disaster's consequence can result in that all the information that has been collected by sensor nodes within a disaster area, cannot be read out any more and is consequently lost.
It has already been considered to avert this danger by a certain redundancy when storing data, for example, by forming groups of several neighbored clusters. In this case, the data aggregated within a cluster of the group is stored by the aggregator node of the cluster itself and in addition by a further aggregator node of another cluster of the respective group. A problem with this approach is in particular the data security, because the data has to be stored persistently over a longer period, i.e. in case of a non real-time network at least over the time span between two query responses.